In recent years, the market of mobile communication systems such as cellular phones has been expanding, and the functionality provided by the service thereof has been becoming sophisticated. Along with this development, the frequency bands used for the mobile communications are gradually shifting toward higher frequency bands of gigahertz (GHz) or higher and, at the same time, tend to use multi-channels. In addition to this, a future possibility of the introduction of Software-Defined-Radio (SDR) technologies is actively discussed.
In the meantime, a tunable high-frequency device using MEMS (Micro Electro Mechanical Systems) technologies is attracting attention. A MEMS device (micromachine device) utilizing the MEMS technologies makes it possible to attain a high Q (quality factor) and can be applied to a variable filter etc. operating in a high frequency band (Japanese Laid-open Patent Publication No. 2008-278147; D. Peroulis et al, “Tunable Lumped Components with Applications to Reconfigurable MEMS Filters”, 2001 IEEE MTT-S Digest, p 341-344; E. Fourn et al., “MEMS Switchable Interdigital Coplanar Filter”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 1, p 320-324, January 2003; and A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p 1878-1885, July 2003). Further, the MEMS device, because of its small size and low loss, is often used in a CPW (Coplanar Waveguide) distributed constant resonator.
“A. A. Tamijani et al, “Miniature and Tunable Filters Using MEMS Capacitors”, IEEE Trans. Microwave Theory Tech., vol. 51, NO. 7, p 1878-1885, July 2003” discloses a filter having a structure in which a plurality of variable capacitors based on MEMS device straddle three distributed constant lines. In this filter, a control voltage Vb is applied to a driving electrode of the MEMS device to thereby displace variable capacitors, vary gaps between the variable capacitors and distributed constant lines, and as a result vary the capacitance. As the capacitance changes, the pass band of the filter changes. For example, by changing the control voltage in a range between 0 and 80 V, the pass band of the filter changes in a range between 21.5 and 18.5 GHz.
However, according to the conventional filter as discussed above, although it is possible to vary the center frequency of the pass band, the bandwidth of the pass band can not be varied.